The term 'abstract art' - also called "non-objective art", "non-figurative", "non-representational", "geometric abstraction", or "concrete art" - is a rather vague umbrella term for any painting or sculpture which does not portray recognizable objects or scenes.
The basic premise of abstraction - incidentally, a key issue of aesthetics - is that the formal qualities of a painting (or sculpture) are just as important (if not more so) than its representational qualities.
One of the most distinct styles of geometric abstract painting to emerge from the modernist era, was the Op-Art movement (an abbreviation of 'optical art') whose hallmark was the engagement of the eye, by means of complex, often monochromatic, geometric patterns, to cause it to see colours and shapes that were not actually there.